1. Field of the Invention
The present invention relates to a photoelectric encoder that emits light from a light source to a main scale and an index scale that can relatively move with respect to each other and obtains a light-receiving signal by interaction between the main scale and the index scale. More particularly, the present invention relates to a photoelectric encoder that has an output signal less affected by a gap change, allows for easy positional adjustment, and has good misalignment characteristics.
2. Description of the Related Art
A photoelectric encoder that emits light from a light source to a main scale and an index scale that can relatively move with respect to each other and obtains a light-receiving signal by interaction between the main scale and the index scale is conventionally used for precise measurement of linear displacement, angular displacement, and the like.
One type of the photoelectric encoder is a so-called two-grating type, shown in FIG. 1. The two-grating type photoelectric encoder uses a scale (optical) grating 21 provided on a main scale 20 and an index (optical) grating 31 provided on a light-transmitting index scale 30. In FIG. 1, a light-receiving element is denoted with 32 and a light-receiving portion is denoted with 34.
In the two-grating type photoelectric encoder, as a gap G between the scale grating 21 and the index grating 31 becomes smaller, effects of diffusion of light and the like become smaller and therefore resolution of the encoder is improved. When the gap G between the gratings is small (e.g., 10 to 50 μm), however, adjustment for positioning the main scale 20 and the index scale 30 is difficult (i.e., misalignment characteristics are not good). Therefore, assembly of the encoder takes a lot of efforts. Moreover, there is a limitation to reduce the gap G between the gratings.
On the other hand, in a so-called three-grating type photoelectric encoder disclosed in Japanese Patent Laid-Open Publication No. 2003-279383, another (optical) grating (called as a first grating) 11 is further arranged between a light source portion 4 including a light-emitting diode (LED) 6 and a collimator lens 8 and the main scale 20 (at a different location from the index grating 31 on the index scale 30 in this example), as shown in FIG. 2. FIG. 2 shows a reflective type as an example. Light from the light source portion 4 is diffracted by the first grating 11. The diffracted light from the first grating 11 is further diffracted by the scale grating (called as a second grating) 21 provided on the main scale 20. Interference fringes generated by diffraction at the scale grating 21 are received by the light-receiving element 32 of the light-receiving portion 34 through the index grating (called as a third grating) 31. As shown in FIG. 2, the light source portion 4, the index scale 30, and the light-receiving portion 34 are provided in a sensor head 2 that can relatively move with respect to the main scale 20.
It is known that an intensity distribution of the interference fringes observed at the light-receiving element 32 does not depend on the gap G between the gratings (i.e., a gap between the first grating 11 and the second grating 21 and a gap between the third grating 31 and the second grating 21). Thus, the gap G between the gratings can be made larger in the three-grating type, as compared with the two-grating type. Thus, adjustment for positioning the scales can be performed more easily in the three-grating type than in the two-grating type (i.e., the three-grating type has better misalignment characteristics than the two-grating type).
However, even if the gap G between the optical gratings can be made larger, adjustment for positioning the scales is still difficult (i.e., misalignment characteristics are not good), because an output signal is largely affected by a gap change and margin of an error of the gap from a designed value of the gap is small.
A photoelectric encoder using a light-receiving element array as the light-receiving portion, in which an index grating and a light-receiving element are integrated with each other and the light-receiving element is arranged in a form of a grating, as described in the description of Japanese Patent No. 2610624, also has the above same problem.